Voltage regulators are known that can convert from input voltages above, below, or equal to the controlled output voltage, respectively performing buck mode regulation, boost mode regulation, or buck-boost mode regulation. Regulator architecture typically is provided for power supplies in automotive applications, lap-top computers, telecom equipment and distributed power systems. A known “four-switch” buck-boost converter is described in an October 2001 datasheet for the LTC3440 “Micro-power Synchronous Buck-Boost DC/DC Converter” integrated circuit manufactured by Linear Technology Corporation. Two of the four switches are connected to the input side of an inductor, the other switches connected to the output side. In accordance with the level of voltage output to be controlled and the level of voltage input, the regulator has the capability of assuming a plurality of operation states in which the switches variously are sequentially activated or deactivated, to connect the inductor to the input, the output, and/or a common ground connection. The voltage mode control technique used presents difficulty in compensating for boost mode and buck-boost mode closed loop operation.
Other known arrangements are simplifications of the “four-switch” configuration in which two of the switches are replaced by diodes. With such arrangements, control flexibility is lessened as fewer different switch operation states are available. A known variable frequency control technique can be used to apply constant-on time control for buck mode and constant-off time control for boost mode. This technique utilizes a wide switching frequency range and a low system bandwidth. Another known alternative is current mode control, wherein a sense resistor is placed permanently in series with the circuit inductor or two sense resistors are used, one at the input and another at the output.
Light load efficiency is important for battery applications. During conditions of low inductor current, activation of switches in regulators, such as described above, at times becomes unnecessary. Without provision in those systems for identifying and adjusting for such conditions, needless switching operations will occur. A need thus exists for switching regulators that eliminate unnecessary switching, thereby to improve efficiency.
During light load conditions in which the output voltage level is greater than the input voltage, regulation proceeds in a discontinuous current boost mode. As inductor current information is not available during certain switching periods, undesirable negative current levels can occur. A need exists to prevent overcharge at the output.